Laser Doppler Vibrometry (LDV) is a well-known non-contact method to measure vibrations or velocity on the surface of a target object. Non-contact measurement of surface velocity can be done using an optical interference method and sensing the Doppler shift of reflected light from a target object. If the target vibration is to be measured, the reflected light picks up a spectrum of sidebands from the range of vibrational frequencies at the target.
Laser Doppler Vibrometer based measurement devices have been widely used in various applications including medicine such as blood flow measuring, customs examination of suspicious vehicles, distant examination of large structures such as buildings, viaducts, bridges, pipes or cores of nuclear power plants, remote listening of facilities for surveillance purposes, land mine detection, diagnosis of fresco paintings. Therefore, LDVs have proven to be useful in a wide variety of applications and technological fields for non-contact, non-destructive, non-invasive evaluation of various structures.
Laser Doppler Vibrometers are optical devices utilizing two coherent laser beams from a single laser source: one as a probe beam and the other is a reference beam. Probe beam is used to illuminate a point on a reflecting target object. Light reflected and/or scattered back is captured to be optically interfered with reference beam so that an interference signal is obtained which represents a Doppler shift in the reflected probe beam. This interference signal is then used to extract the velocity and displacement, i.e. vibration, information of the point on the target object surface. Such a system mainly comprises of a Bragg Cell, photodetector, demodulator and filtering modules in order to achieve the goal of converting reflected laser beam into a baseband analog signal.
Generally, the preferred method of laser Doppler vibrometry shifts the sensor's time varying signals to a higher frequency to eliminate velocity directional ambiguity and to lessen the additional electronic noise typically found near zero frequency (DC). Many commercially available LDVs accomplish this by using an optical frequency offset in a frequency-shifted self-heterodyne (or offset homodyne) detection method. Typically, this offset is provided by putting the reference optical beam through an acousto-optic modulator (AOM) crystal to shift the optical carrier frequency. When this reference beam is combined with the reflected beam from the target onto a photodiode, the time-varying heterodyne signal containing the target's vibrational modulation is centered about this intermediate frequency offset. Usually, to obey the physical constraints of the AOM, this frequency is tens of MHz or higher. Often, this high intermediate frequency is electrically mixed to yet lower frequencies for subsequent frequency demodulation, in order to extract the target's vibrational velocity spectrum. To avoid electrical down-conversion, and to obtain a smaller frequency offset, sometimes two AOMs are used in tandem, with large opposite shifts at a differential frequency. Usually such vibrometers use free-space optical beams or more compact forms of fiber coupled systems.
There are two interferometric methods conventionally used for LDV applications: homodyne detection and heterodyne detection. The heterodyne detection method using frequency shifting techniques overcomes a number of drawbacks inherent in homodyne detection.
Devices comprising a single-beam LDV system in concert with a beam scanning system have also been developed. Scanned single-beam techniques are suitable for measuring vibrations that are repetitive (e.g., continuously cycling over the same location); however, because the measurements are made sequentially from one location to the next, the value of this technique is limited when the vibrations are transient or non-repetitive or have continuous components comprising relatively high wide band-width. Measurement of non-repetitive vibrations is important when analyzing civil structures, aerospace composite components, and for buried land mine detection. While a plurality of single-beam LDV systems could be used to measure multiple locations on an object, this would be a costly and complicated option if a large number of simultaneous measurements were required.
Simultaneous measurement of multiple locations on an object is needed in order to gain more complete information on an object's vibrational characteristics. Specifically, simultaneous LDV measurements yield: (a) phase information among the measured points, (b) increased inspection speed, and (c) the ability to measure non-repetitive vibration patterns. A simultaneous multi-beam LDV system based on a homodyne interferometer design has also been investigated. However, because that multi-beam technique is based on a homodyne detection method, it is affected by the same performance limitations as the single-beam homodyne system described above.
In view of the foregoing, there is a need in the art for an LDV device that can simultaneously measure multiple locations on an object with the benefits of high signal-to-noise ratio, wide dynamic range, and high accuracy inherent with heterodyne detection.
Devices and methods are developed for remotely detecting sounds using laser Doppler vibrometers. These devices direct a laser probe beam at a point on a reflecting surface to acquire sounds in the proximity. Speech signals and other acoustical signals at the target space cause reflecting surface to vibrate. This vibration modulates the reflected probe beam in accordance with the Doppler Effect. Total acoustical signal is then obtained by the laser Doppler vibrometer, i.e., interference signal is converted into an analog electrical signal, demodulated and filtered to extract sound signals. If the device is used for eavesdropping purposes probe beam is preferred to be invisible and reflecting surface is usually a window glass in a target room. Although such devices are given different names as laser-bounce listening devices, remote laser voice-detection systems etc., they are usually called as laser listening devices.
Laser listening devices are advantageous over microphones. Such a device can be directed at reflecting surfaces in the proximity of sound sources under surveillance from hundreds of meters while microphones are required to be placed in the monitored space. Laser listening devices can avail the surveillance of a facility with adequate intelligibility of speech and even when a comprehensible speech cannot be acquired, an essence of the matter, e.g., number, genders of persons, can be understood without access to the monitored space.
In general, main drawback of laser listening devices arises from the vibrating properties of target surfaces. Sound sources in a monitored space generate vibrations on the surface of an object; therefore, the surface itself acts as a transducer. Hence, an analogy can be drawn between the surface and a microphone, but contrary to a microphone membrane, vibrating properties of a relatively large target surface is modal or inhomogeneous. In other words, whole surface or points on it do not tend to exhibit flat transfer functions in contrast to microphones. Furthermore, target objects with higher dimensions and larger mass have poor vibration properties causing degradation of the signal integrity to be monitored. Each point on surface carries different vibration information of the signal around the vicinity and lacking information (a frequency component) in one point may exhibit itself in another. Therefore, it is always beneficial to acquire signals from multiple points on a surface to be able to defragment information. Laser listening devices which avail monitoring from multiple points by scanning a target surface require separate detectors and/or very fast CCD cameras which make the prior art expensive and complicated.
Another potential problem for laser Doppler vibrometers as listening devices is the background noise: background noise originating from machines and other sources around the vicinity cause vibrations on the surface resulting in contamination of signals under surveillance. Consequently, extracting audible signals through a laser listening device with a single probe beam directed at a single point may become a highly challenging task due to poorly vibrating target surfaces and background noises. On the other hand, noise problem can be overcome to some extent by applying various online/offline signal processing methods as filtering.
It would be therefore desirable to have relatively cost effective and simple device and methods that allows monitoring sounds from multiple points on a reflecting target surface. It would also be beneficial taking the advantage of having plurality of signals to be able to apply advanced and multi-channel noise reduction or signal separation techniques as blind source separation, active noise cancellation or even Wiener filtering.